Morrison had nothing new in particular to promote, and there were no expectations before his performance. He is notoriously moody and cranky, as capable of mumbling and mailing it in as he is of transcendence. His records in the 1980s and '90s were spotty and often seemed like less than his best effort.

In Portland that night, with much of the crowd still out in the concourse, Morrison came out and burned. He opened with "Baby Please Don't Go" and "Here Comes the Night," two songs from his early days with Them, and didn't let up, pausing so briefly between songs that his set ended up an 80-minute medley. He stalked around the stage in dark glasses and a black porkpie hat and growled through an extended "Raincheck" before ending with "Gloria." Dylan's subdued effort felt like an afterthought.

The concert (which has been bootlegged) was a reminder that when Morrison turns it on, there's no singer in modern music who can touch him. "As a physical fact, Morrison may have the richest and most expressive voice pop music has produced since Elvis Presley, and a sense of himself as an artist that Presley was always denied," Greil Marcus writes in "When That Rough God Goes Riding: Listening to Van Morrison." "But what is that voice for?"

It's a good question, one Marcus spends most of his short, maddening book trying to answer. Morrison always has been about being in the moment and trying to hold what Irish tenor John McCormack called "the yarragh in your voice." Marcus makes much of the yarragh, and as an abstract concept it's not a bad way to think about what Morrison does. His phrasing can appear random and instinctive and he thinks nothing of holding a note or repeating a line until he's good and ready to let it go. On "Madame George," the great track on "Astral Weeks," he repeats "dry your eye" 13 times and "the love that loves to love" until it collapses in the corner. On "Listen to the Lion" he roars and growls and purrs the beast into submission.

Marcus aptly describes Morrison as "small and gloomy, a burly man with more black energy than he knew what to do with, the wrong guy to meet in a dark alley, or backstage on the wrong night." Morrison's stubborn artistic independence and willingness to look foolish in pursuit of his genius makes him a good match for Marcus and makes "When That Rough God Goes Riding" a bookend to the author's "Like a Rolling Stone: Bob Dylan at the Crossroads."

As a critic, Marcus is high-toned, obtuse and snobbish. He's also literate, brainy and fearless in making cross-genre comparisons. He has a deep knowledge of Americana but can be sloppy with details. (Morrison's song is "Linden Arden Stole the Highlights," not "Arlen.") Marcus dismisses 15 years of Morrison's output with the wave of his hand and has essays on the use of Morrison's music in the movies "Breakfast on Pluto" and "Georgia." (And what about that surprising version of Pink Floyd's "Comfortably Numb" in "The Departed"?)